This invention relates to photoelectric sensors of the type for non-contact sensing of the lateral position (side-to-side displacement) of an object, an edge of that object, or a pair of edges of that object (thickness variations), in a sensing region. More particularly, this invention relates to through-beam sensors of the aforementioned type wherein the sensing region is located between a light source and a detector. Still more particularly, the invention relates to photoelectric sensors of the aforementioned type which realize improved signal strength and stability and provide continuous sensing and indication of the aforesaid position throughout the entire sensing region.
In manufacturing process control, it is often necessary to monitor a running length of material such as a fiber or web to detect lateral excursion of the fiber or web from the desired path or to detect thickness of a fiber. Due to fragility of the fiber or web, such sensing is customarily accomplished by non-contact detection such as with a through-beam photoelectric sensor. Sensors of this type typically use an LED emitter and a simple single output phototransistor or photodiode detector. The difference in output of such detectors between fully blocked and unblocked conditions of the beam is small, particularly when detecting small diameter fibers that block only a portion of the beam and permit a majority of the beam to impinge the detector. As a result, such detectors are susceptible to problems caused by the affects of contamination, electrical noise and drift. Moreover, such sensors do not provide a continuous indication of the position of a small diameter fiber when the same is fully within the light beam and moving between the edges of the beam.
Photodiode arrays, also called line arrays, have been used in through-beam sensing applications. Photodiode arrays have a multiplicity of photodiodes (often 512) arranged in a line. The outputs of the individual diodes are multiplexed and the time sequence of the output relates position of light (and shadow) on the array. The required multiplexing circuit adds cost to the sensor and has too long a response time for high speed sensing due to the length of time required to sequentially read the responses of the individual diodes of the array.
A position-sensitive detector (PSD) provides a substantially stronger and more stable output signal and a fast response time, making it very suitable for industrial control applications. A PSD is an optoelectronic sensor that responds to a light spot impinging its surface, providing continuous position data of that spot as the spot travels along the surface. However, through-beam sensor applications work on the principle of projecting a shadow on the detector, responding to light intensity. Heretofore, PSDs have not been employed in through-beam sensor applications.